Dehydrogenation process using layered catalyst composition

ABSTRACT

This invention relates to a dehydrogenation process using a layered catalyst composition. The catalyst composition comprises an inner core such as alpha-alumina, and an outer layer bonded to the inner core composed of an outer refractory inorganic oxide such as gamma-alumina. The outer layer has uniformly dispersed thereon a platinum group metal such as platinum and a promoter metal such as tin. The composition also contains a modifier metal such as lithium. The catalyst composition shows improved durability and selectivity for dehydrogenating hydrocarbons, especially at dehydrogenation conditions comprising a low water concentration.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of prior copendingU.S. application Ser. No. 09/887,229, filed Jun. 22, 2001, which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] This invention relates to a hydrocarbon dehydrogenation processusing a layered catalyst composition at select operating conditions forincreased catalyst stability.

BACKGROUND OF THE INVENTION

[0003] Platinum based catalysts are used for numerous hydrocarbonconversion processes. In many applications promoters and modifiers arealso used. One such hydrocarbon conversion process is thedehydrogenation of hydrocarbons, particularly alkanes such as isobutanewhich are converted to isobutylene. For example, U.S. Pat. No. 3,878,131(and related U.S. Pat. Nos. 3,632,503 and 3,755,481) discloses acatalyst comprising a platinum metal, a tin oxide component and agermanium oxide component. All components are uniformly dispersedthroughout the alumina support. U.S. Pat. No. 3,761,531 (and relatedU.S. Pat. No. 3,682,838) discloses a catalytic composite comprising aplatinum group component, a Group IVA (IUPAC 14) metallic component,e.g., germanium, a Group VA (IUPAC 15) metallic component, e.g.,arsenic, antimony, and an alkali or alkaline earth component alldispersed on an alumina carrier material. Again all the components areevenly distributed on the carrier.

[0004] U.S. Pat. No. 6,177,381 describes a dehydrogenation process usinga layered catalyst composition. Example 7 of U.S. Pat. No. 6,177,381describes testing of Catalysts A, B, E, and F for dehydrogenationactivity using a hydrocarbon feed. A water concentration of 2000 ppmbased on hydrocarbon weight was injected. The deactivation rates ofCatalysts A, B, E, and F were 0.052, 0.032, 0.050, and 0.033° F/hr,respectively.

[0005] Although these deactivation rates are relatively low, otherdehydrogenation processes are sought that have even lower deactivationrates.

SUMMARY OF THE INVENTION

[0006] An improved dehydrogenation process using a layered catalystcomposition which exhibits excellent stability at a critical combinationof catalyst properties and operating conditions is disclosed. When thethickness of the outer layer of the layered catalyst is in the range offrom 40 to 150 microns, the loading of the platinum group metal in theentire layered catalyst is in the range of from about 5 to about 30gram-mole of the platinum group metal per cubic meter of the entirelayered catalyst, and the concentration of the platinum group metal inthe outer layer of the layered catalyst is from about 0.026 to about0.26 gram-mole of the platinum group metal per kilogram of the outerlayer, then excellent stability results, provided that the amount ofwater passed to the layered catalyst is less than 1000 wt-ppm, andpreferably less than 300 wt-ppm, and more preferably less than 60 wt-ppmbased on the amount of hydrocarbon passed to the layered catalyst. Thisresult was unexpected because previously it had been thought that suchhigh amounts of platinum-group metal in the outer layer would adverselyaffect stability. However, it is now recognized that evendehydrogenation processes using catalysts that have relatively highamounts of platinum-group metal in the outer layer can be operated atlow water concentrations and thus achieve excellent catalyst stability.

[0007] In addition, the process disclosed exhibits better selectivitythan processes of the prior art, in terms of total selectivity to normalolefins.

[0008] In a broad embodiment, this invention is a hydrocarbondehydrogenation process comprising contacting a hydrocarbon stream witha layered composition under dehydrogenation conditions to give adehydrogenated product. The layered composition comprises an inner coreand an outer layer bonded to the inner core. The outer layer comprisesan outer refractory inorganic oxide and has a thickness of from about 40to about 150 microns. The outer layer also has, uniformly dispersedthereon, at least one platinum group metal and at least one promotermetal, where the concentration of the at least one platinum group metalin the outer layer is from about 0.026 to about 0.26 gram-mole of theplatinum group metal on an elemental basis per kilogram of the outerlayer. The layered composition has a loading of the at least oneplatinum group metal of from about 5 to about 30 gram-mole of theplatinum group metal on an elemental basis per cubic meter of thelayered composition. The layered composition further has dispersedthereon at least one modifier metal. The inner core and the outerrefractory inorganic oxide comprise different materials. Thedehydrogenation conditions comprise a weight of water passed to thelayered composition based on the hydrocarbon weight passed to thelayered composition of less than 1000 ppm.

[0009] Other objects and embodiments are described in the detaileddescription of the invention.

INFORMATION DISCLOSURE

[0010] U.S. Pat. No. 6,177,381 describes a dehydrogenation process usinga layered catalyst composition. The entire teachings of U.S. Pat. No.6,177,381 are incorporated herein by reference.

DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention relates to a dehydrogenation process thatuses layered catalyst composition. The layered catalyst compositioncomprises an inner core composed of a material which has substantiallylower adsorptive capacity for catalytic metal precursors, relative tothe outer layer. Some of the inner core materials are also notsubstantially penetrated by liquids, e.g., metals including but notlimited to aluminum, titanium and zirconium. Examples of the inner corematerial include, but are not limited to, refractory inorganic oxides,silicon carbide and metals. Examples of refractory inorganic oxidesinclude without limitation alpha alumina, theta alumina, cordierite,zirconia, titania and mixtures thereof. A preferred inorganic oxide isalpha alumina.

[0012] These materials which form the inner core can be formed into avariety of shapes such as pellets, extrudates, spheres or irregularlyshaped particles although not all materials can be formed into eachshape. Preparation of the inner core can be done by means known in theart such as oil dropping, pressure molding, metal forming, pelletizing,granulation, extrusion, rolling methods and marumerizing. A sphericalinner core is preferred. The inner core whether spherical or not has aneffective diameter of about 0.05 mm to about 5 mm and preferably fromabout 0.8 mm to about 3 mm. For a non-spherical inner core, effectivediameter is defined as the diameter the shaped article would have if itwere molded into a sphere. Once the inner core is prepared, it iscalcined at a temperature of about 400° C. to about 1500° C.

[0013] The inner core is now coated with a layer of a refractoryinorganic oxide which is different from the inorganic oxide which may beused as the inner core and will be referred to as the outer refractoryinorganic oxide. This outer refractory oxide is one which has goodporosity, has a surface area of at least 50 m²/g, and preferably atleast 150 m²/g, an apparent bulk density of about 0.2 g/ml to about 1.0g/ml and is chosen from the group consisting of gamma alumina, deltaalumina, eta alumina, theta alumina, silica/alumina, zeolites,non-zeolitic molecular sieves (NZMS), titania, zirconia and mixturesthereof. It should be pointed out that silica/alumina is not a physicalmixture of silica and alumina but means an acidic and amorphous materialthat has been cogelled or coprecipitated. This term is well known in theart, see e.g., U.S. Pat. Nos. 3,909,450; 3,274,124; and 4,988,659, allof which are incorporated by reference. Examples of zeolites include,but are not limited to, zeolite Y, zeolite X, zeolite L, zeolite beta,ferrierite, MFI, mordenite and erionite. Non-zeolitic molecular sieves(NZMS) are those molecular sieves which contain elements other thanaluminum and silicon and include silicoaluminophosphates (SAPOs)described in U.S. Pat. No. 4,440,871, ELAPOs described in U.S. Pat. No.4,793,984, MeAPOs described in U.S. Pat. No. 4,567,029 all of which areincorporated by reference. Preferred refractory inorganic oxides aregamma and eta alumina.

[0014] A preferred way of preparing a gamma alumina is by the well-knownoil drop method which is described in U.S. Pat. No. 2,620,314 which isincorporated by reference. The oil drop method comprises forming analuminum hydrosol by any of the techniques taught in the art andpreferably by reacting aluminum metal with hydrochloric acid; combiningthe hydrosol with a suitable gelling agent, e.g.,hexamethylenetetraamine; and dropping the resultant mixture into an oilbath maintained at elevated temperatures (about 93° C.). The droplets ofthe mixture remain in the oil bath until they set and form hydrogelspheres. The spheres are then continuously withdrawn from the oil bathand typically subjected to specific aging and drying treatments in oiland ammoniacal solutions to further improve their physicalcharacteristics. The resulting aged and gelled spheres are then washedand dried at a relatively low temperature of about 80° C. to 260° C. andthen calcined at a temperature of about 455° C. to 705° C. for a periodof about 1 to about 20 hours. This treatment effects conversion of thehydrogel to the corresponding crystalline gamma alumina.

[0015] The layer is applied by forming a slurry of the outer refractoryoxide and then coating the inner core with the slurry by means wellknown in the art. Slurries of inorganic oxides can be prepared by meanswell known in the art which usually involve the use of a peptizingagent. For example, any of the transitional aluminas can be mixed withwater and an acid such as nitric, hydrochloric, or sulfuric to give aslurry. Alternatively, an aluminum sol can be made by for example,dissolving aluminum metal in hydrochloric acid and then mixing thealuminum sol with the alumina powder.

[0016] It is also required that the slurry contain an organic bondingagent which aids in the adhesion of the layer material to the innercore. Examples of this organic bonding agent include but are not limitedto polyvinyl alcohol (PVA), hydroxy propyl cellulose, methyl celluloseand carboxy methyl cellulose. The amount of organic bonding agent whichis added to the slurry will vary considerably from about 0.1 wt-% toabout 3 wt-% of the slurry. How strongly the outer layer is bonded tothe inner core can be measured by the amount of layer material lostduring an attrition test, i.e., attrition loss. Loss of the secondrefractory oxide by attrition is measured by agitating the catalyst,collecting the fines and calculating an attrition loss. It has beenfound that by using an organic bonding agent as described above, theattrition loss is less than about 10 wt-% of the outer layer. Finally,the thickness of the outer layer varies from about 40 to about 150microns. One micron equals 10-6 meter.

[0017] Depending on the particle size of the outer refractory inorganicoxide, it may be necessary to mill the slurry in order to reduce theparticle size and simultaneously give a narrower particle sizedistribution. This can be done by means known in the art such as ballmilling for times of about 30 minutes to about 5 hours and preferablyfrom about 1.5 to about 3 hours. It has been found that using a slurrywith a narrow particle size distribution improves the bonding of theouter layer to the inner core.

[0018] Without wishing to be bound by any particular theory, it appearsthat bonding agents such as PVA aid in making an interlocking bondbetween the outer layer material and the inner core. Whether this occursby the PVA reducing the surface tension of the core or by some othermechanism is not clear. What is clear is that a considerable reductionin loss of the outer layer by attrition is observed.

[0019] The slurry may also contain an inorganic bonding agent selectedfrom an alumina bonding agent, a silica bonding agent or mixturesthereof. Examples of silica bonding agents include silica sol and silicagel, while examples of alumina bonding agents include alumina sol,boehmite and aluminum nitrate. The inorganic bonding agents areconverted to alumina or silica in the finished composition. The amountof inorganic bonding agent varies from about 2 to about 15 wt-% as theoxide, and based on the weight of the slurry.

[0020] Coating of the inner core with the slurry can be accomplished bymeans such as rolling, dipping, spraying, etc. One preferred techniqueinvolves using a fixed fluidized bed of inner core particles andspraying the slurry into the bed to coat the particles evenly. Thethickness of the layer can vary considerably, but usually is from about40 to about 150 microns. It should be pointed out that the optimum layerthickness depends on the choice of the outer refractory oxide. Once theinner core is coated with the layer of outer refractory inorganic oxide,the resultant layered support is dried at a temperature of about 100° C.to about 320° C. for a time of about 1 to about 24 hours and thencalcined at a temperature of about 400° C. to about 900° C. for a timeof about 0.5 to about 10 hours to effectively bond the outer layer tothe inner core and provide a layered catalyst support. Of course, thedrying and calcining steps can be combined into one step.

[0021] When the inner core is composed of a refractory inorganic oxide(inner refractory oxide), it is necessary that the outer refractoryinorganic oxide be different from the inner refractory oxide.Additionally, it is required that the inner refractory inorganic oxidehave a substantially lower adsorptive capacity for catalytic metalprecursors relative to the outer refractory inorganic oxide.

[0022] Having obtained the layered catalyst support, catalytic metalscan be dispersed on the layered support by means known in the art. Thus,a platinum group metal, a promoter metal and a modifier metal can bedispersed on the outer layer. The platinum group metals includeplatinum, palladium, rhodium, iridium, ruthenium and osmium. Promotermetals are selected from the group consisting of tin, germanium,rhenium, gallium, bismuth, lead, indium, cerium, zinc and mixturesthereof, while modifier metals are selected from the group consisting ofalkali metals, alkaline earth metals and mixtures thereof.

[0023] These catalytic metal components can be deposited on the layeredsupport in any suitable manner known in the art. One method involvesimpregnating the layered support with a solution (preferably aqueous) ofa decomposable compound of the metal or metals. By decomposable is meantthat upon heating the metal compound is converted to the metal or metaloxide with the release of byproducts. Illustrative of the decomposablecompounds of the platinum group metals are chloroplatinic acid, ammoniumchloroplatinate, bromoplatinic acid, dinitrodiamino platinum, sodiumtetranitroplatinate, rhodium trichoride, hexa-amminerhodium chloride,rhodium carbonylchloride, sodium hexanitrorhodate, chloropalladic acid,palladium chloride, palladium nitrate, diamminepalladium hydroxide,tetraamminepalladium chloride, hexachloroiridate (IV) acid,hexachloroiridate (III) acid, ammonium hexachloroiridate (III), ammoniumaquohexachloroiridate (IV), ruthenium tetrachloride,hexachlororuthenate, hexa-ammineruthenium chloride, osmium trichlorideand ammonium osmium chloride. Illustrative of the decomposable promotermetal compounds are the halide salts of the promoter metals. A preferredpromoter is tin and preferred decomposable compounds are stannouschloride or stannic chloride.

[0024] The alkali and alkaline earth metals which can be used asmodifier metals in the practice of this invention include lithium,sodium, potassium, cesium, rubidium, beryllium, magnesium, calcium,strontium and barium. Preferred modifier metals are lithium, potassium,sodium and cesium with lithium and sodium being especially preferred.Illustrative of the decomposable compounds of the alkali and alkalineearth metals are the halide, nitrate, carbonate or hydroxide compounds,e.g., potassium hydroxide, lithium nitrate.

[0025] All three types of metals can be impregnated using one commonsolution or they can be sequentially impregnated in any order, but notnecessarily with equivalent results. A preferred impregnation procedureinvolves the use of a steam-jacketed rotary dryer. The support isimmersed in the impregnating solution containing the desired metalcompound contained in the dryer and the support is tumbled therein bythe rotating motion of the dryer. Evaporation of the solution in contactwith the tumbling support is expedited by applying steam to the dryerjacket. The resultant composite is allowed to dry under ambienttemperature conditions, or dried at a temperature of about 80° C. toabout 350° C., followed by calcination at a temperature of about 200° C.to about 700° C. for a time of about 1 to about 4 hours, therebyconverting the metal compound to the metal or metal oxide. It should bepointed out that for the platinum group metal compound, it is preferredto carry out the calcination at a temperature of about 400° C. to about700° C.

[0026] In one method of preparation, the promoter metal is firstdeposited onto the layered support and calcined as described above andthen the modifier metal and platinum group metal are simultaneouslydispersed onto the layered support by using an aqueous solution whichcontains a compound of the modifier metal and a compound of the platinumgroup metal. The support is impregnated with the solution as describedabove and then calcined at a temperature of about 400° C. to about 700°C. for a time of about 1 to about 4 hours.

[0027] An alternative method of preparation involves adding one or moreof the metal components to the outer refractory oxide prior to applyingit as a layer onto the inner core. For example, a decomposable salt ofthe promoter metal, e.g., tin (IV) chloride can be added to a slurrycomposed of gamma alumina and aluminum sol. Further, either the modifiermetal or the platinum group metal or both can be added to the slurry.Thus, in one method, all three catalytic metals are deposited onto theouter refractory oxide prior to depositing the second refractory oxideas a layer onto the inner core. Again, the three types of catalyticmetals can be deposited onto the outer refractory oxide powder in anyorder although not necessarily with equivalent results.

[0028] Another method of preparation involves first impregnating thepromoter metal onto the outer refractory oxide and calcining asdescribed above. Next, a slurry is prepared (as described above) usingthe outer refractory oxide containing the promoter metal and applied tothe inner core by means described above. Finally, the modifier metal andplatinum group metal are simultaneously impregnated onto the layeredcomposition which contains the promoter metal and calcined as describedabove to give the desired layered catalyst.

[0029] As a final step in the preparation of the layered catalystcomposition, the catalyst composition is reduced under hydrogen or otherreducing atmosphere in order to ensure that the platinum group metalcomponent is in the metallic state (zero valent). Reduction is carriedout at a temperature of about 1 00° C. to about 650° C. for a time ofabout 0.5 to about 10 hours in a reducing environment, preferably dryhydrogen. The state of the promoter and modifier metals can be metallic(zero valent), metal oxide or metal oxychloride.

[0030] The layered catalyst composition can also contain a halogencomponent which can be fluorine, chlorine, bromine, iodine or mixturesthereof with chlorine and bromine preferred. This halogen component ispresent in an amount of 0.03 to about 1.5 wt-% with respect to theweight of the entire catalyst composition. The halogen component can beapplied by means well known in the art and can be done at any pointduring the preparation of the catalyst composition although notnecessarily with equivalent results. It is preferred to add the halogencomponent after all the catalytic components have been added eitherbefore or after treatment with hydrogen.

[0031] Although in the preferred embodiments all three metals areuniformly distributed throughout the outer layer of outer refractoryoxide and substantially present only in the outer layer, it is alsowithin the bounds of this invention that the modifier metal can bepresent both in the outer layer and the inner core. This is owing to thefact that the modifier metal can migrate to the inner core, when thecore is other than a metallic core.

[0032] Although the concentration of each metal component can varysubstantially, it is desirable that the platinum group metal be presentin a concentration of about 0.01 to about 5 weight percent on anelemental basis of the entire weight of the catalyst and preferably fromabout 0.05 to about 2.0 wt-%. The promoter metal is present in an amountfrom about 0.05 to about 10 wt-% of the entire catalyst while themodifier metal is present in an amount from about 0.1 to about 5 wt-% ofthe entire catalyst. Finally, the atomic ratio of the platinum groupmetal to promoter metal varies from about 0.05 to about 5. In particularwhen the promoter metal is tin, the atomic ratio is from about 0.1:1 toabout 5:1 and preferably from about 0.5:1 to about 3:1. When thepromoter metal is germanium the ratio is from about 0.25:1 to about 5:1and when the promoter metal is rhenium, the ratio is from about 0.05:1to about 2.75:1.

[0033] In addition, the layered catalyst for use in the process of thisinvention has a critical concentration of the platinum group metal inthe outer layer. This concentration is generally from about 0.026 toabout 0.26 gram-mole of the platinum group metal, on an elemental basisper kilogram of the outer layer. When the platinum group metal isplatinum, this concentration is from about 0.5 to about 5 wt-% ofplatinum on an elemental basis and based on the weight of the outerlayer. For a given concentration of the platinum group metal in theouter layer, there is a preferred atomic ratio of the platinum groupmetal to the promoter metal. For example, when the platinumconcentration is between about 0.5 and about 3 wt-% of platinum on anelemental basis and based on the weight of the outer layer, thepreferred atomic ratio of platinum to tin is from between about 0.6:1 toabout 1.3:1, increasing as the platinum concentration increases.

[0034] Suitable catalysts generally have a loading of the platinum groupmetal of from about 5 to about 30 gram-mole of the platinum group metalon an elemental basis per cubic meter of the layered catalyst. When theplatinum group metal is platinum, this loading is from about 0.0010 toabout 0.0060 gram of platinum on an elemental basis per cubic centimeterof catalyst.

[0035] The concentration of the platinum-group metal in the outer layercan be readily determined in at least three ways. First, theconcentration can be computed based on the weight of the ingredientsused in preparing the layered catalyst. Second, in the case where thelayered catalyst has previously been prepared and the inner refractoryinorganic oxide is different from the outer refractory inorganic oxide,then the inner layer refractory inorganic oxide can be separated fromthe outer refractory inorganic oxide, and the platinum group metal canbe separately recovered, by known chemical and/or mechanical methods.Then, the concentration of the weight of the platinum group metal can bedetermined from the weight of recovered platinum group metal and theweight of recovered inner refractory inorganic oxide. Finally, energydispersive x-ray spectroscopy or wavelength dispersive spectroscopy(EPMA) using a scanning electron microscope of a sample of the layeredcatalyst may also be used.

[0036] Having obtained the layered catalyst, it can be used in ahydrocarbon dehydrogenation process. It is critical that the process ofthis invention be carried out at certain conditions in order to achievethe surprising benefit of improved catalyst stability.

[0037] Dehydrogenatable hydrocarbons are contacted with the catalyst ofthe instant invention in a dehydrogenation zone maintained atdehydrogenation conditions. This contacting can be accomplished in afixed catalyst bed system, a moving catalyst bed system, a fluidized bedsystem, etc., or in a batch-type operation. A fixed bed system ispreferred. In this fixed bed system the hydrocarbon feed stream ispreheated to the desired reaction temperature and then flowed into thedehydrogenation zone containing a fixed bed of the catalyst. Thedehydrogenation zone may itself comprise one or more separate reactionzones with heating means there between to ensure that the desiredreaction temperature can be maintained at the entrance to each reactionzone. The hydrocarbon may be contacted with the catalyst bed in eitherupward, downward or radial flow fashion. Radial flow of the hydrocarbonthrough the catalyst bed is preferred. The hydrocarbon may be in theliquid phase, a mixed vapor-liquid phase or the vapor phase when itcontacts the catalyst. Preferably, it is in the vapor phase.

[0038] Hydrocarbons which can be dehydrogenated include hydrocarbonswith 2 to 30 or more carbon atoms including normal paraffins,isoparaffins, alkylaromatics, naphthenes and olefins. A preferred groupof hydrocarbons is the group of normal paraffins with 2 to about 30carbon atoms. Especially preferred normal paraffins are those having 9to 16 carbon atoms. Other especially preferred paraffins are monomethylparaffins and dimethyl paraffins having from 9 to 16 carbon atoms. Eachof the aforementioned hydrocarbons may be present alone or in a mixturewith one or more of any of the other aforementioned hydrocarbons.

[0039] Dehydrogenation conditions include a temperature of from about400° C. to about 900° C., a pressure of from about 1 to about 1013 kPaand a liquid hourly space velocity (LHSV) of from about 0.1 to about 100hr⁻¹. As used herein, the abbreviation ‘LHSV’ means liquid hourly spacevelocity, which is defined as the volumetric flow rate of liquid perhour divided by the catalyst volume, where the liquid volume and thecatalyst volume are in the same volumetric units. Generally forparaffins, the lower the molecular weight, the higher is the temperaturerequired for comparable conversion. The pressure in the dehydrogenationzone is maintained as low as practicable, consistent with equipmentlimitations, to maximize the chemical equilibrium advantages.

[0040] The effluent stream from the dehydrogenation zone generally willcontain unconverted dehydrogenatable hydrocarbons, hydrogen and theproducts of dehydrogenation reactions. This effluent stream is typicallycooled and passed to a hydrogen separation zone to separate ahydrogen-rich vapor phase from a hydrocarbon-rich liquid phase.Generally, the hydrocarbon-rich liquid phase is further separated bymeans of either a suitable selective adsorbent, a selective solvent, aselective reaction or reactions or by means of a suitable fractionationscheme. Unconverted dehydrogenatable hydrocarbons are recovered and maybe recycled to the dehydrogenation zone. Products of the dehydrogenationreactions are recovered as final products or as intermediate products inthe preparation of other compounds.

[0041] The dehydrogenatable hydrocarbons may be admixed with a diluentmaterial before, while or after being flowed to the dehydrogenationzone. The diluent material may be hydrogen, steam, methane, ethane,carbon dioxide, nitrogen, argon and the like or a mixture thereof.Hydrogen is the preferred diluent. Ordinarily, when hydrogen is utilizedas the diluent it is utilized in amounts sufficient to ensure a hydrogento hydrocarbon mole ratio of about 0.1:1 to about 40:1, with bestresults being obtained when the mole ratio range is about 1:1 to about10:1. The diluent hydrogen stream passed to the dehydrogenation zonewill typically be recycled hydrogen separated from the effluent from thedehydrogenation zone in the hydrogen separation zone.

[0042] Water or a material which decomposes at dehydrogenationconditions to form water such as an alcohol, aldehyde, ether or ketone,for example, may be added to the dehydrogenation zone, eithercontinuously or intermittently, in an amount to provide, calculated onthe basis of equivalent water, less than about 1000 weight ppm of thehydrocarbon feed stream, preferably less than 300 weight ppm, morepreferably less than 60 weight ppm, and possibly even less than 1 weightppm. The process of this invention may be operated with no water ormaterial which decomposes to form water added to the dehydrogenationzone.

[0043] The following examples are presented in illustration of thisinvention and are not intended as undue limitations on the generallybroad scope of the invention as set out in the appended claims.

EXAMPLE 1

[0044] A catalyst was prepared essentially according to the methoddescribed in Example 1 of U.S. application Ser. No. 09/887,229, excepton a different scale.

[0045] Alumina spheres were prepared by the well-known oil drop method,which is described in U.S. Pat. No. 2,620,314 which is incorporated byreference. This process involves forming an aluminum hydrosol bydissolving aluminum in hydrochloric acid. Hexamethylene tetraamine wasadded to the sol to gel the sol into spheres when dispersed as dropletsinto an oil bath maintained at about 93° C. The droplets remained in theoil bath until they set and formed hydrogel spheres. After the sphereswere removed from the hot oil,,they were pressure-aged at about 135° C.and washed with dilute ammonium hydroxide solution, dried at about 110°C. and calcined at about 650° C. for about 2 hours to give gamma aluminaspheres. The calcined alumina was then crushed into a fine powder havinga particle size of less than 200 microns.

[0046] Next, a slurry was prepared by mixing aluminum sol (15 wt-%Al₂O₃) and deionized water and agitated to uniformly distribute the tincomponent. To this mixture there were added the above prepared aluminapowder and a 50% aqueous solution of tin(IV) chloride, and the slurrywas ball milled for approximately 240 minutes thereby reducing themaximum particle size to less than 50 microns. This slurry was sprayedonto cordierite cores having an average diameter of about 1.8 mm byusing a granulating and coating apparatus to give an outer layer ofabout 120 microns. At the end of the process, some of the slurry wasleft which did not coat the cores. This layered spherical support wasdried at about 200° C. and then calcined at about 600° C. in order toconvert the pseudoboehmite in the outer layer into gamma alumina andconvert the tin chloride to tin oxide.

[0047] The calcined layered support was impregnated with lithium andplatinum using a rotary impregnator by contacting the support with anaqueous solution (1:1 solution:support volume ratio) containing lithiumchloride and chloroplatinic acid based on support weight. Theimpregnated composite was heated using a rotary impregnator until nosolution remained, dried at about 315° C. and calcined at about 540° C.and reduced in hydrogen at about 500° C.

[0048] The resulting catalyst prepared in this example contained 0.158wt-% platinum, 0.15 wt-% tin, and 0.22 wt-% lithium with respect to theentire catalyst. This catalyst was identified as catalyst A. Theproperties of catalyst A are summarized in Table 1. The tinconcentration in the layer was computed from the tin content of theslurry. The platinum concentration in the layer was computed bymultiplying the tin concentration in the layer by the weight ratio ofplatinum to tin with respect to the entire catalyst.

EXAMPLE 2

[0049] A catalyst was prepared essentially according to the methoddescribed in Example 2 of U.S. application Ser. No. 09/887,229, excepton a different scale. The impregnated composite was dried at 315° C. andcalcined at 540° C. The resulting catalyst prepared in this example hada layer thickness of about 55 micron, and contained 0.553 wt-% platinum,0.32 wt-% tin and 0.14 wt-% lithium. This catalyst was identified ascatalyst B. The properties of catalyst B are summarized in Table 1.TABLE 1 Catalyst Identification A B Core Cordierite Cordierite CoreDiameter (in) 0.0673 0.0618 Layer Thickness (micron) 120 55 TinConcentration in Layer 0.65 1.97 (wt % tin) Platinum Concentration inLayer 0.68 3.4 (wt % platinum) Platinum Concentration in Layer 0.0350.17 (g-mole platinum per kilogram layer) Platinum Loading 0.012 0.043(g platinum per 10 cc catalyst) Platinum Loading 6.1 22.0 (g-moleplatinum per cubic meter catalyst) Atomic Ratio of Platinum to Tin 0.641.05

EXAMPLE 3

[0050] Catalyst B was tested for dehydrogenation activity in alaboratory scale plant. In a 1.27 cm (½″) reactor, 10 cc of catalyst wasplaced and a hydrocarbon feed composed of 8.8-9.3 wt-% n-C₁₀, 40.0-41.8wt-% n-C₁₁, 38.6 wt-% n-C₁₂, 8.6-10.8 wt-% n-C₁₃, 0.3-0.8 wt-% n-C₁₄ and1-1.4 wt-% non-normals was flowed over the catalyst under a pressure of138 kPa(g) (20 psi(g)), a H₂:hydrocarbon molar ratio of 6:1, and aliquid hourly space velocity (LHSV) of 20 hr⁻¹. The total normal olefinconcentration in the product (% TNO) was maintained at 15 wt-% byadjusting reactor temperature.

[0051] Recycle hydrogen and hydrocarbon feed were combined upstream ofthe reactor to form a combined feed, and the combined feed was vaporizedprior to entering the reactor. In this example, the catalyst was testedat water concentrations of 2000, 300, and 40-60 wt-ppm based on theweight of the hydrocarbon in the combined feed. In order to achieve thedesired water concentration of 2000 wt-ppm the recycle hydrogen wasrouted through a water bath, and the temperature and pressure of thewater were set as needed. In order to achieve the desired waterconcentrations of 40-60 and 300 wt-ppm an alcohol that readilydecomposes at dehydrogenation conditions was injected into the combinedfeed. If the water concentration of the recycle hydrogen or hydrocarbonfeed was too high to achieve any of these water concentrations, therecycle hydrogen or hydrocarbon feed was dried by routing through amolecular sieve. The results of the testing are presented in the Table 2below. What is presented is the deactivation rate (slope) which isobtained by plotting temperature (° F.) needed to maintain 15% TNO(total normal olefins) versus time. The data show a substantial decreasein catalyst deactivation rate when operating at water levels of 300wt-ppm or below.

EXAMPLE 4

[0052] Catalyst A was tested in the manner described in Example 3 atwater concentrations of 2000 and 60 wt-ppm based on the weight of thehydrocarbon in the combined feed. The results are shown in the Table 2below. Catalyst A also shows a significant decrease in deactivation ratewhen operating at a water level of 60 wt-ppm. However this benefit isnot as substantial as the benefit seen in Example 3.

EXAMPLE 5

[0053] Catalyst A was tested in the manner described in Example 3, butwith modified operating conditions. While the liquid hourly spacevelocity (LHSV) remained 20 hr⁻¹, the H₂:hydrocarbon molar ratio wasdropped to 4:1. Water concentrations of 3000 and 1000 wt-ppm based onthe hydrocarbon in the combined feed were tested. These waterconcentrations were achieved by routing the recycle hydrogen through awater bath and controlling the temperature and pressure of the bath. Theresults are shown in the Table 2 below.

EXAMPLE 6

[0054] Catalyst A was tested in the manner described in Example 5, butwith a liquid hourly space velocity (LHSV) of 30 hr⁻¹ and aH₂:hydrocarbon molar ratio of 8:1. The results are shown in the Table 2below.

[0055] In both Examples 5 and 6, the deactivation rate at 3000 wt-ppmwas similar to that at 1000 wt-ppm. This implies that there is little orno change in catalyst stability between 3000 and 1000 wt-ppm waterconcentration. It is only below 1000 wt-ppm that an improvement incatalyst stability (a decrease in deactivation rate) is seen. TABLE 2Hydrogen/ Hydrocarbon Water Deactivation Ex- LHSV Ratio ConcentrationRate ample Catalyst (hr⁻¹) (mol/mol) (wt-ppm) (° F./hr) 3 B 20 6 20000.092  300 0.018 40-60 0.016 4 A 20 6 2000 0.096  60 0.05  5 A 20 4 30000.212 1000 0.198 6 A 30 8 3000 0.132 1000 0.142

What is claimed is:
 1. A hydrocarbon dehydrogenation process comprisingcontacting a hydrocarbon stream with a layered composition underdehydrogenation conditions to give a dehydrogenated product, the layeredcomposition comprising an inner core, an outer layer bonded to the innercore, the outer layer comprising an outer refractory inorganic oxide andhaving a thickness of from about 40 to about 150 microns and havinguniformly dispersed thereon at least one platinum group metal and atleast one promoter metal and having a concentration of the at least oneplatinum group metal of from about 0.026 to about 0.26 gram-mole of theplatinum group metal on an elemental basis per kilogram of the outerlayer, the layered composition further having dispersed thereon at leastone modifier metal, the inner core and the outer refractory inorganicoxide being different materials, the layered composition further havinga loading of the at least one platinum group metal of from about 5 toabout 30 gram-mole of the platinum group metal on an elemental basis percubic meter of the layered composition, the dehydrogenation conditionscomprising a weight of water passed to the layered composition of lessthan 1000 ppm based on the hydrocarbon weight passed to the layeredcomposition.
 2. The process of claim 1 wherein the weight of waterpassed to the layered composition is less than 300 ppm.
 3. The processof claim 1 wherein the weight of water passed to the layered compositionis less than 60 ppm.
 4. The process of claim 1 further characterized inthat the dehydrogenation conditions comprise a temperature of about 400to about 900° C. and a pressure of about 1 to about 1013 kPa.
 5. Theprocess of claim 1 wherein the inner core is selected from the groupconsisting of alpha alumina, metals, theta alumina, silicon carbide,cordierite, zirconia, titania and mixtures thereof.
 6. The process ofclaim 1 wherein the outer refractory inorganic oxide is selected fromthe group consisting of gamma alumina, delta alumina, theta alumina,silica/alumina, zeolites, nonzeolitic molecular sieves, titania,zirconia and mixtures thereof.
 7. The process of claim 1 wherein theplatinum group metal is selected from the group consisting of platinum,palladium, rhodium, iridium, ruthenium, osmium and mixtures thereof. 8.The process of claim 1 wherein the promoter metal is selected from thegroup consisting of tin, germanium, rhenium, gallium, bismuth, lead,indium, cerium, zinc and mixtures thereof.
 9. The process of claim 1wherein the modifier metal is selected from the group consisting ofalkali metals, alkaline earth metals and mixtures thereof.
 10. Theprocess of claim 1 wherein the hydrocarbon stream comprises at least oneC₂-C₃₀ hydrocarbon selected from the group consisting of normalparaffins, isoparaffins, alkylaromatics, naphthenes, and olefins. 11.The process of claim 1 wherein the hydrocarbon stream comprises normalparaffins having 2 to 15 carbon atoms.
 12. The process of claim 1wherein the hydrocarbon stream comprises monomethyl paraffins ordimethyl paraffins.
 13. A hydrocarbon dehydrogenation process comprisingcontacting a hydrocarbon stream with a layered composition underdehydrogenation conditions to give a dehydrogenated product, the layeredcomposition comprising an inner core, an outer layer bonded to the innercore, the outer layer comprising an outer refractory inorganic oxide andhaving a thickness of from about 40 to about 150 microns and havinguniformly dispersed thereon platinum and tin and having a concentrationof platinum of from about 0.5 to about 5 wt-% of platinum on anelemental basis and based on the weight of the outer layer, the layeredcomposition further having dispersed thereon lithium, the inner core andthe outer refractory inorganic oxide being different materials, thelayered composition further having a loading of platinum metal of fromabout 0.0010 to about 0.0060 gram of platinum on an elemental basis percubic centimeter of the layered composition, the dehydrogenationconditions comprising a weight of water passed to the layeredcomposition of less than 1000 ppm based on the hydrocarbon weight passedto the layered composition.
 14. The process of claim 13 wherein theweight of water passed to the layered composition is less than 300 ppm.15. The process of claim 13 further characterized in that thedehydrogenation conditions comprise a temperature of about 400° C. toabout 900° C. and a pressure of about 1 kPa to about 1013 kPa.
 16. Theprocess of claim 13 wherein the inner core is selected from the groupconsisting of alpha alumina, metals, theta alumina, silicon carbide,cordierite, zirconia, titania and mixtures thereof.
 17. The process ofclaim 13 wherein the outer refractory inorganic oxide is selected fromthe group consisting of gamma alumina,,delta alumina, theta alumina,silica/alumina, zeolites, nonzeolitic molecular sieves, titania,zirconia and mixtures thereof.
 18. The process of claim 13 wherein thehydrocarbon stream comprises a C₉-C₁₅ hydrocarbon selected from thegroup consisting of normal paraffins, monomethyl paraffins, and dimethylparaffins.